The present invention relates to an improved process for preparing polyvinyl halide polymers having increased impact resistance and increased clarity.
It is a common practice to reinforce rigid plastics as polyvinyl chloride, polymethyl methacrylate, polystyrene, styreneacrylonitrile copolymers and the like with particles of rubber polymers such as polybutadiene and the polyacrylates. The addition of rubber to these common plastics improves their impact strength, that is their ability to withstand a rapidly applied shock. While the addition of rubber improves the impact strength of these plastics, their other physical properties such as tensile strength, clarity, heat distortion temperature, hardness and aging stability, are, however, adversely affected by the rubber. In most cases, the addition of the larger amounts of rubber which would produce the maximum amount of impact strength results in a plastic that is too soft for many uses. The common commercial products, then, are a compromise between the desire to increase impact strength while being able to maintain their other necessary physical properties.
Many references exist which describe how to prepare rubber-reinforced plastics. The rubbery particles can be dispersed in the rigid phase by mill blending or latex blending, or, by polymerizing the hard polymer in the presence of the rubber. The rubber has been shown to exist in the hard polymer matrices as discrete particles of about 0.1 to 5 microns in diameter. See U.S. Pat. Nos. 3,632,679 and 3,644,576.
U.S. Pat. No. 3,661,994 describes the preparation, by means of seeded emulsion polymerization procedures, of multilayered or so-called "sandwich" polymeric particles having a hard inner core and a rubbery outer layer. The latter disclosure notes that such sandwich particles can, per se be blended with rigid plastics or they can, first, have an additional layer of a hard polymer grafted thereon whereupon they may then be blended with rigid plastics. In either case, the thus modified rigid plastics display substantially improved impact strength without deleteriously affecting any other physical properties, such as tensile strength, as has been known to occur when conventionally prepared rubber particles are employed as impact resistance additives for rigid plastics.
However, one of the disadvantages of the latter multilayered or sandwich particles relates to the fact that their outer rubber layer is applied by means of an emulsion polymerization procedure which results in the final particles having a relatively small particle size which, in commercial practice, usually necessitates their isolation by means of a costly, time consuming spray-drying technique or by coagulation with a brine solution which is also time consuming and which may introduce ionic impurities into the polymer. Needless to say, it would be highly desirable to find some means of eliminating such inefficient product isolation and drying procedures.
British Pat. No. 1,015,334 discloses a method for preparing a process aid which, if a crosslinking agent is included, some impact modification may be recognized. The basic process requires polymerization of an acrylate in the presence of a porous vinyl chloride.
Basically, however, all of these procedures have numerous disadvantages in order to achieve the impact strength advantage. The emulsion system requires isolation by means of a costly, time consuming spray drying technique, or by coagulation with a brine solution. The emulsion system also generally requires multiple reaction systems which tie up plant production. Either the spray drying or the coagulation introduces impurities into the polymer. The multilayered sandwich system requires extra steps and is more costly to operate from that standpoint. The process of the British patent can only utilize a very small amount of crosslinking agent, i.e., from 0.01 to 10%, based on the total weight of the acrylates, and preferably only 0.05-1.5%. Needless to say, it would be highly desirable to find some means of preparing polyvinyl halide resins which have improved impact resistance without the inefficiencies of the prior art.
It is an object of the invention to prepare polymer particles which are substantially pure and free from polymerization contaminants, which are easier to handle, which have improved clarity, impact, and reduced haze, by means of an improved process which can be accomplished in a single reaction vessel.